The present invention is generally related to the manufacture of implants for medical purposes. More specifically, the present invention provides an implant made of UHMW polyethylene, intended for sterilization by means of .gamma. rays or electron beams, which is manufactured in a way that inhibits oxidation, thus diminishing the implant's susceptibility to long-term brittleness and preventing increased wear at contact locations.
Only a few companies have specialized in the manufacture of UHMW polyethylene for medical purposes. A usual manufacture process uses a powder or a granulate which, at temperatures around 180.degree. C.-240.degree. C. and pressures around 2-10 MPa, is compressed into finished parts or extruded into blocks or bars from which implant parts (e.g., bearing shells of artificial hip joints or artificial knee joints) are manufactured.
A further processing step that is common in the manufacture of implant parts consists of welding the parts in a protective gas atmosphere (e.g., nitrogen), placing the implants into bags and sterilizing the implants in such bags via irradiation with .gamma. rays or electron beams. Once sterilized, the implant parts can be stored and be readily available for use.
However, .gamma. ray or electron beam irradiation not only sterilizes the implant part, but causes chain structures with free radicals to arise in the polyethylene, which leads to cross linking and/or oxidation in the presence of oxygen atoms. Oxygen is present in the polyethylene and can also diffuse slowly into the polyethylene, which causes the material to become brittle after several years, which reduces the mechanical properties of the implant and leads to increased wear.
Prior art has reported the stabilization of polyolefins against thermal oxidation and photo-oxidation which is not damaging to health. Such art shows various polyolefins age more slowly under natural environmental conditions if a tocopherol is mixed with them during the manufacturing process. In contrast to other antioxidants, such as derivatives of phenolamines and phosphides, which can be hazardous to health, it has been proposed that tocopherol be mixed with polyolefins, which are used in the food industry, health services or as implants, in order to achieve slower aging. Reports show that weight proportions of 0.01% to 5% of tocopherol in the polyolefins slow the thermal oxidation during the manufacture of the material and the photo-oxidation of the material under natural environmental conditions.
In light of the above, it would be desirable to provide implants made of UHMW polyethylene, and sterilized by means of irradiation with .gamma. rays or electron beams, which had oxidation inhibiting characteristics, thus increasing the useful life of the implant.